The invention relates to a device for determining a characteristic point in the cardiac cycle. Such a device can be used for, for example, activating an intra-aortal balloon pump (IABP).
An IABP contains, inter alia, an intra-aortal balloon (IAB), which can be inserted, for example, into the aorta of a patient with a poorly functioning heart, and a pumping device.
In each cardiac cycle the IAB is inflated by means of the pumping device after the end of an ejection phase of the left ventricle of the heart, and is deflated again before the commencement of the following ejection phase.
The pumping action of the heart is improved in this way, and there is an improvement in the blood supply to the coronary artery.
For good functioning of the IABP it is of great importance for the IAB to be inflated and deflated at the correct times in the cardiac cycle. In particular, the correct choice of the time at which the IAB is inflated is of very great importance.
If the IAB is inflated too soon, the pumping action of the heart is reinforced to a lesser extent, or the pumping action can even be adversely affected, because the prematurely inflated IAB causes a flow resistance in the aorta during the ejection of the left ventricle which is still occurring at the time.
If the time selected is too late, the functioning of the IABP is also less effective. A lower volume of blood is then pumped through the IAB, and the coronary artery and the vascular bed undergo a high perfusion pressure for only a short period of time.
The times for inflating and deflating the IAB can be set manually by an experienced person at fixed times in the cardiac cycle on the basis of the electro-cardiogram (ECG) of the heart. A disadvantage of this is that when there is a gradual acceleration or slowing-down of the cardiac cycle the set times deviate increasingly from the desired times, and therefore have to be reset repeatedly. It is also impossible to make allowances for an irregular cardiac cycle, and in particular the setting of the time at which the IAB is inflated is not performed sufficiently accurately.
The end of the ejection phase and the accompanying closure of the aortic valve are themselves indicated accurately by the occurrence of a dip in the arterial blood pressure signal P(t). This dip is also known as the incisura point.
U.S. Pat. No. 5,183,051 discloses a device by means of which an attempt is made to determine the incisura point by looking for the dip in the curve of the arterial blood pressure signal P(t) within a previously defined period of time. However, the period of time may be incorrectly defined and, besides, the device does not work in the case of a damped blood pressure signal, because in that case the incisura point is not accompanied by a clear blood pressure change.
A further disadvantage of this device is that it is still not possible to make allowance for an irregular cardiac cycle while patients in whom an IABP is used generally have an irregular cardiac cycle. Moreover, the use of the device for activating an IABP is not mentioned at all in U.S. Pat. No. 5,183,051.
A device which detects the incisura point in the curve of the arterial blood pressure signal P(t) is proposed in IEEE Transactions on Biomedical Engineering 1990, 37(2), pp. 182-192. However, it is possible that this device may interpret irregularities in the curve of the arterial blood pressure signal as the incisura point, which upsets the functioning of the IABP.
U.S. Pat. No. 4,809,681 discloses a device for activating an IABP which determines from the ECG the point at which the IAB must be deflated. However, it is not possible to determine the incisura point using the device.
Sakamoto et al., ASAIO Journal 1995, pp. 79-83, discloses a device which forecasts the position of the incisura point in a cardiac cycle from the ECG by calculating the length of the ejection phase from the period of time of the previous heartbeat. This device is still inaccurate.